You'll have less income through ToG if you're casting a lot of combat spells. Without ToG, once you factor in drafting and training +10% more military and paying more wages a human's not really competitive with a dwarf or orc in terms of cost per acre to train and build in a war situation. Then again, mystics usually sacrifice some economy to their magic capabilities. Also, the human's spellbook isn't great compared to an elf. Even a dwarf has fool's gold, but humans have no unique combat spells.